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LTVT_v0_21_3MC.exe
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Jim Mosher
jimmosher@yahoo.com
21 April 2010

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General Description
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"LTVT_v0_21_3MC.exe" is a modification of version "LTVT_v0_21_3.exe" of the 
Lunar Terminator Visualization Tool:

  http://ltvt.wikispaces.com

produced at the request of Maurice Collins, and permitting DEM simulations to be drawn 
with "constant" illumination.

In the Geometry section in the upper right of the main screen, a checkbox has been
added next to the "Sub-solar point" heading.  Checking this box changes the
meaning of the input boxes under it:

  Sub-solar longitude -->  Sun azimuth (clock angle measured clockwise from lunar north)
  Sub-solar latitude  -->  Sun angle (elevation of Sun above local horizontal)

Unlike in the normal simulations where the incident light has a fixed direction in
space, giving an effective solar azimuth and elevation that varies with surface position,
in the simulations produced when this box is checked, the light striking every point on
the surface is assumed to arrive at the same specified solar azimuth and elevation.

Sun angle azimuths (like sub-solar longitudes) can be entered with no restriction of range.
Adding or subtracting multiples of 360 has no effect on the sun angle.  They are reckoned 
clockwise from the northerly direction of the meridian of constant longitude passing through 
the point of interest. 

  Azimuth =   0 or  360 (or similar multiples) means lighting from the north
  Azimuth =  90 or -270 or the equivalent means lighting from the east
  Azimuth = -90 or  270 or the equivalent means lighting from the west
  Azimuth = 180 or -180 means lighting from the south

Sun angle elevations (like sub-solar latitudes), on the other hand, are restricted 
to the range -90 to +90.  They are reckoned relative to the local radial direction.

  Elevation = -90 indicates lighting coming "up" from the planet's center
  Elevation = 0 means lighting coming from a direction at right angles to the local radial
  Elevation = +90 means lighting coming down on the surface parallel to the local radial

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Notes
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1. Since the "lighting" simulated in the constant sun angle mode differs from any 
obtainable in the real world, it is recommended that the surface brightness be evaluated 
using the simple "Lambertian" reflectance model (under "Tools...Change DEM options...").

2. The definition of solar azimuths with respect to the planetary north direction restricts 
the simulations to one of many conceivable kinds of "constant" illumination.  In this 
particular system, looking down on the north or south pole, the constant sun angle shadows 
circulate around the pole.  Although "LTVT_v0_21_3MC.exe" contains no provision for specifying 
a different system, if an aziumth reference other than the north pole could be selected a somewhat 
different pattern of shadows would be obtained for the same sun angle.


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Limitations
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1. When the special checkbox is unchecked, this version of LTVT should, in all respects,
be identical to "LTVT_v0_21_3.exe"; but when the box is checked, none of the functions
other than the 2D and 3D simulations should be expected to work correctly, and even
for them some of the readouts may be erroneous.

2. For all purposes other than the drawing of DEM simulations the two input boxes 
retain their normal meaning of sub-solar longitude and latitude, no matter how they
they may be labeled based on the current status of the checkbox.  

3. In particular, even if labeled as solar azimuth and elevation, the two input boxes
will be treated as sub-solar longitude and latitude for purposes of determining the 
location of the red-blue terminator lines (if present), and for generating the sun angle 
information displayed for the location pointed to by the mouse in the mouse-over data 
output area in the lower right of the main screen. 

4. For example, the varying "sun angle" indicated in the mouse-over display area is
obviously erroneous for the DEM simulations drawn with constant sun angle.



